1. Field of the Invention
The present invention generally relates to plant genetics. More specifically, the invention relates to genes involved in the biosynthesis of condensed tannins, and methods for use thereof.
2. Description of the Related Art
Condensed tannins (CTs), also known as proanthocyanidins, are flavonoid polymers that have a long history of use as tanning agents for animal skins and as determinants of flavor and astringency in teas, wines and fruit juices. The chemistry of proanthocyanidins has been studied for decades. Their name reflects the fact that, on acid hydrolysis, the extension units are converted to colored anthocyanidins, and this forms the basis of the classical assay for these compounds (Porter 1989).
The building blocks of most proanthocyanidins are (+)-catechin and (−)-epicatechin. (−)-Epicatechin has 2,3-cis stereochemistry and (+)-catechin has 2,3-trans-stereochemistry. These stereochemical differences are of major importance in proanthocyanidin biosynthesis, since all chiral intermediates in the flavonoid pathway up to and including leucoanthocyanidin are of the 2,3-trans stereochemistry, raising important questions about the origin of the 2,3-cis stereochemistry of (−)-epicatechin, the commonest extension unit in proanthocyanidins (Foo and Porter 1980).
Recently, sensitive and specific methods, utilizing HPLC and mass spectrometry, have been developed for the fractionation and identification of proanthocyanidins, with accurate determination of both amount and degree of polymerization of the different sized oligomeric fractions (Cheynier et al., 1999; Gu et al., 2002). Armed with this technology, the detailed proanthocyanidin profiles and compositions were recently determined for over 40 common food sources (Gu et al., 2004). The foods with the highest levels of total proanthocyanidins were, in decreasing order, ground cinnamon, sorghum (sumac bran), dry grape seed, unsweetened baking chocolate, raw pinto beans, sorghum (high tannin whole grain), choke berries, red kidney beans, hazelnuts and pecan nuts (Gu et al., 2004).
Condensed tannins are present in many plants and are oligomers or polymers of flavonoid (flavan-3-ol) units. CTs are also commonly termed proanthocyanidins due to the red anthocyanidins that are produced upon heating in acidic alcohol solutions. The most common anthocyanidins produced are cyanidin (from procyanidin) and delphinidin (from prodelphinidin). CTs may contain from 2 to 50 or more flavonoid units. CT polymers have complex structures because of variations in the flavonoid units and the sites for interflavan bonds. Depending on their chemical structure and degree of polymerization, CTs may or may not be soluble in aqueous organic solvents.
CTs are attracting increasing attention due to their ability to affect the nutritional quality of human and animal food (Bagchi et al., 2000; Barry and McNabb, 1999; Morris and Robbins, 1997). In addition, CTs from various plants have beneficial effects on cardiac health and immune responses (Pataki et al., 2002; Foo et al., 2000; Lin et al., 2002). They can reversibly bind to proteins and reduce their degradation rate. The presence of moderate amounts of CT in forage crops reduces the initial rate of microbial digestion of the protein component of forage material in the rumen. The protein tannin complexes then pass to the abomasum where they dissociate at the lower pH, providing “by-pass protein” for utilization by the animal and consequent enhancement of milk and wool production and live weight gain (Barry and McNabb, 1999; Tanner et al., 1995).
In Arabidopsis thaliana, CTs accumulate predominantly in the endothelium layer of the seed coat. Many mutations that affect the seed coat color and CT accumulation have been characterized and the corresponding genes cloned. These genes include BAN (Genbank Accession No. AF092912; Devic et al., 1999), TTG1 TTG2, TT1, TT2, TT8 and TT12. The ban mutation leads to accumulation of anthocyanins in the seed coat instead of CTs. TTG1, TTG2, TT1, TT2 and TT8 are regulatory genes and encode a WD-repeat protein (Walker et al., 1999), a WRKY family protein, a WIP subfamily plant zinc finger protein (Sagasser et al., 2002), an R2R3 MYB domain protein (Nesi et al, 2001) and a basic helix-loop-helix domain protein (Nesi et al., 2000), respectively. Expression of the TT2 gene has been shown to induce the TT8 and BAN genes but did not lead to accumulation of CT in Arabidopsis (Nesi et al., 2001). All the above genes are predominantly expressed in the seed coat endothelium.
The foregoing studies have provided a further understanding of the metabolism of plant secondary metabolism. However, the prior art has failed to provide techniques for the application of this understanding to the creation of plants having valuable new characteristics. What are thus needed are practical techniques for the production of novel plants with improved phenotypes and methods for the use thereof. Such techniques may allow the creation and use of plants with improved nutritional quality, thereby benefiting both human and animal health and representing a substantial benefit in the art.